GB 2127711 describes a process for degassing water using a recirculating inert gas, such as nitrogen, which is regenerated and purified, in the gaseous state. The water is preferably seawater that is to be used as injection water in underground oil reservoirs to obtain a higher degree of hydrocarbon recovery. After contacting the water with the nitrogen gas, the free oxygen transferred to the nitrogen gas is catalytically reacted to form steam. The catalytic reaction is performed in a conversion vessel filled with dry, granulated catalyst such as Pd or Pt on alumina over which is passed the free oxygen-containing stripper gas and pure (99.9%) hydrogen. The pure hydrogen is provided by a water electrolyzer.
EP 0234771 describes an adaptation of this process using a plurality of treatment stages each including a treatment zone through which are passed in co-current flow the water and inert gas. Whereas pure hydrogen may be used for catalytically converting free oxygen in the inert gas into steam, because heat exchange means may be provided to heat the reductant, other reductants may be used such as methanol or natural gas, in particular methanol.
WO 2004/069753 describes yet another adaptation of this process wherein a free oxygen containing inert stripping gas (nitrogen) is used to combust dispersed hydrocarbons in the so-called “produced water” recovered in the production of oil & gas.
Using hydrocarbons to combust free oxygen in stripping gas requires operation of the catalyst at high temperatures, e.g. >300° C. High temperature operation of the catalyst is undesirable as it shortens catalyst life and consumes large amounts of energy. Furthermore, catalyst poisons such as sulphur compounds are often present in hydrocarbon mixtures, especially those recovered from crude oil or natural gas. Methanol combusts more cleanly but cannot generally be produced locally to the de-gassing operation and so must be stored. Methanol is a toxic and highly flammable liquid and storage, particularly in off-shore operations, poses a number of technical and safety difficulties. Electrolytic production of pure hydrogen remains inefficient and expensive. Furthermore, local storage of hydrogen, e.g. in cylinders again poses a number of technical and safety problems.
Therefore there is a need to provide an efficient process for reducing the free oxygen content of nitrogen stripping gas.